Fatigue Calculations of Multi-Mode VIV

Pipelines on undulating seabed may be subject to free-span areas where interaction between VIV response of several modes occurs, including both single and multi-spans. Within the project Ormen Lange a model test program, as completed in 2004, was performed to investigate this phenomena. The implementation of the test results into detail design went via an Ormen Lange specific design guideline in which the general analysis flow is given. However, taking the mode interaction effects from the tests into the actual routing makes the need for additional evaluations and generalisation on dynamic system of interacting spans. A dynamic system represents a set of spans were the eigenmodes interact physically and not only mathematically in computation of the eigenmodes over a long pipeline section. For a single span case we find that the modes are well separated while for multi-span problems modes are close in frequencies. A clear criterion and limitation on dynamic system to be considered becomes vital. The relative effect of inline and cross flow VIV in actual design depends on the SN-curves at hand. The general trend is that inline generated fatigue takes over for corrosion sensitive problems covering operation phase, whereas preliminary phases may be governed by cross flow response. The modelling of pipe-soil interaction at shoulders is vital for the multi-span response characteristics, the experience being that softer soil gives minor span interaction. The soft soil gives deeper pipeline penetration and more uniform support stiffness all along the shoulder than the hard soil for which local reactions occur more like pinned supports.